In bowl, combine brown sugar, flour, 1/2 cup cocoa, baking soda, baking powder, cinnamon and salt.
In separate bowl, beat together egg and egg white; beat in buttermilk, coffee and oil. Make a well in sugar mixture; pour coffee mixture into well and whisk just till blended.
Pour into 2 greased 8" square cake pans. Bake in 350°F oven for 30 to 40 minutes or until a toothpick inserted into centre comes out clean and cake springs back when pressed. Let cool in pan on rack for 20 minutes.
Meanwhile; in small saucepan, combine remaining cocoa, sugar and cornstarch. Whisk in milk until blended, then corn syrup and vanilla. Bring to boil over medium-high heat, whisking constantly, reduce heat to low and cook, whisking for 1 to 2 minutes or until thickened and glossy. Let cool for 5 minutes. Spread evenly over cake.